The disclosure relates generally to hot gas path components, and more particularly, to a coupon for replacing a cutout of a hot gas path component. In one embodiment, the coupon includes a number of manufacturing assist features for improved coupling and finishing of the coupon. In another embodiment, the coupon includes cooling openings configured to not be blocked by a coating applied to an outer surface of the coupon.
Hot gas path components are used in turbomachines to direct a working fluid in a manner to create energy. Hot gas path components can take a variety of forms such as turbomachine blades (rotor blades or stationary vanes) that include airfoils that direct a working fluid to create energy. Rotor blades are coupled to and act to turn a turbine rotor, and stationary vanes are coupled to a casing of the turbomachine to direct the working fluid towards the rotor blades.
Some of the most advanced hot gas path components include near wall cooling configurations to cool outer walls of the components. However, near wall cooling configurations present a significant challenge for manufacturing. In recent years, additive manufacturing, such as direct metal laser melting (DMLM) or selective laser melting (SLM), has emerged as a reliable manufacturing method for such ultra-efficient near wall cooling arrangements. The advent of additive manufacturing techniques has also provided the ability to replace sections of hot gas path components such as a leading or trailing edge of a turbomachine blade. For example, a portion of a leading edge of a turbomachine blade may be removed, leaving a cutout in the blade, and a new section (referred to herein as a “coupon”) may be coupled in the cutout. The coupon can replace a worn section of a used turbomachine blade, or be added as part of a new turbomachine blade. The coupon can simply replace internal cooling structures of the turbomachine blade, or may provide additional or improved cooling structures, e.g., near wall cooling passages, internal cooling passages, impingement sleeves, pin banks, etc., that were not provided in the original turbomachine blade.
Despite the growth of additive manufacturing to create the coupons, the use of coupons to replace sections of hot gas path components presents a number of manufacturing challenges.
In order to unlock the potential of this method for the targeted replacement of larger segments of the component, an exact match of the coupon and the precision machined cutout is needed for achieving reliable joining quality. The gap for joining (gap between coupon and cutout in the component) depends on the precision of the contouring of the coupon and the original component. Any mismatch will result in a variation of the gap distance. Tight gap tolerances with gap widths below 100 μm are required if a narrow gap brazing process is chosen. In order to obtain these tight tolerances and to ensure an optimum fit between both parts, the same machining path is used for the wire electrical discharge machining (EDM) cutting of coupon and original component. In one approach, a wire EDM control program is prepared and used first for the precision machining of the coupon. Thereafter, the same control program is used a second time for the machining of a matching cutout in the hot gas path component. During this second step only the width of the cutting tool (i.e., the EDM wire thickness) is compensated, so that a near ‘zero gap’ fit is obtained between the additive manufactured coupon and the original component. Despite this approach, gaps can still exist between the coupon and the cutout in the original component.
Another challenge for use of coupon replacements is providing precise manual re-contouring after coupon joining. In particular, during the re-contouring of components with cooling channels close to the hot gas surface, minimum wall thickness requirements may be violated and the near wall cooling scheme may be damaged. This is particularly cumbersome as wall thickness tolerances are tight and as there is no direct feedback about the remaining wall thickness for the grinding operator during the grinding process. A minimal wall thickness (e.g., 1.2 millimeter) between a hot gas side and a near wall cooling passage may be required to achieve mechanical integrity and lifetime assessments.
After the joining and re-contouring steps for a coupon are implemented, one or more protective coatings (e.g., a bond coating and a thermal barrier coating) are applied. One obstacle presented by the coating(s) is the unavoidable clogging of the existing cooling passages by coating overspray. Conventionally, after coating(s) application, a reopening step is required for the blocked cooling passages in order to meet the airflow requirements of the hot gas path component. Due to the cooling passage arrangement, this reopening of the near wall cooling passage exit holes must be completed very carefully in order to avoid any risk of clogging of the exit holes. In particular, sharp bending angles in the opening may cause reamer tools to break, creating additional clogging. Consequently, the process is time-consuming and expensive.